The present disclosure relates to optical elements and methods for producing optical elements. Specifically, the present disclosure relates to an optical element that selectively directionally reflects light in at least one specific wavelength range while transmitting light in at least one wavelength range other than the specific wavelength range.
Recently, building glass for high-rise buildings and houses and window glass for vehicles have increasingly been provided with layers for absorbing or reflecting some of sunlight. This is one of the energy-conservation measures for preventing global warming and is aimed at reducing the load on air-conditioning systems that increases as more optical energy from sunlight enters an indoor space through a window to raise the indoor temperature thereof. The optical energy from sunlight is concentrated in the visible region, namely, a wavelength range from 380 to 780 nm, and the near infrared region, namely, a wavelength range from 780 to 2,100 nm. In particular, the transmittance of windows in the near infrared region, which is irrelevant to human visibility, is an important factor that determines the performance of highly transparent, highly heat-shielding windows.
Methods for shielding near infrared light while maintaining sufficient transmittance in the visible region include a method of providing window glass with a layer having high reflectance in the near infrared region and a method of providing window glass with a layer having high absorptance in the near infrared region.
For the first method, numerous techniques have been disclosed that use a reflective layer such as an optical laminated layer, a metal-containing layer, or a transparent conductive layer (see, for example, PCT International Publication No. WO 05/087680). Such a reflective layer, however, can only specularly reflect incident sunlight because the layer is provided on flat window glass. Accordingly, light coming from the sky is specularly reflected to reach another building or the ground, which absorbs the light as heat and therefore raises ambient temperature. This causes problems around a building with windows covered with such reflective layers, including an urban heat-island effect due to a local temperature rise and inhibition of lawn growth in a region irradiated with the reflected light.
For the second method, numerous techniques using organic dye films have been disclosed (see, for example, Japanese Unexamined Patent Application Publication No. 06-299139 and Japanese Patent Application Nos. 09-316115 and 2001-89492). A dye film attached to window glass, however, absorbs light as heat at the surface of the window and transfers some of the heat indoors as radiant heat, thus having problems such as insufficient shielding performance and the risk of the glass cracking due to thermal stress. Another problem is that the dye film has low weather resistance and is inconvenient for use at places where frequent replacement is difficult, such as high-rise buildings.
It is therefore desirable to provide an optical element capable of selectively directionally reflecting light in at least one specific wavelength range while transmitting light in at least one wavelength range other than the specific wavelength range and also to provide a method for producing such an optical element.